A supernova (/ˌsuːpərnoʊvə/ plural: supernovae
/ˌsuːpərnoʊviː/ or supernovas, abbreviations: SN and SNe) is a
transient astronomical event that occurs during the last stellar
evolutionary stages of a massive star's life, whose destruction is
marked by one final titanic explosion. This causes the sudden
appearance of a "new" bright star, before slowly fading from sight
over several weeks or months.

Supernovae are more energetic than novae. In Latin, nova means "new",
referring astronomically to what appears to be a temporary new bright
star. Adding the prefix "super-" distinguishes supernovae from
ordinary novae, which are far less luminous. The word supernova was
coined by
Walter BaadeWalter Baade and
Fritz ZwickyFritz Zwicky in 1931.[1]
Only three
Milky WayMilky Way naked-eye supernova events have been observed
during the last thousand years, though many have been seen in other
galaxies using telescopes. The most recent directly observed supernova
in the
Milky WayMilky Way was
Kepler's SupernovaKepler's Supernova in 1604, but two more recent
supernova remnants have also been found. Statistical observations of
supernovae in other galaxies suggest they occur on average about three
times every century in the Milky Way, and that any galactic supernova
would almost certainly be observable with modern astronomical
telescopes.
Supernovae may expel much, if not all, of the material away from a
star[2] at velocities up to 7007300000000000000♠30,000 km/s or
10% of the speed of light. This drives an expanding and fast-moving
shock wave[3] into the surrounding interstellar medium, and in turn,
sweeping up an expanding shell of gas and dust, which is observed as a
supernova remnant. Supernovae create, fuse and eject the bulk of the
chemical elements produced by nucleosynthesis.[4] Supernovae play a
significant role in enriching the interstellar medium with the heavier
atomic mass chemical elements.[5] Furthermore, the expanding shock
waves from supernovae can trigger the formation of new stars.[6][7]
SupernovaSupernova remnants are expected to accelerate a large fraction of
galactic primary cosmic rays, but direct evidence for cosmic ray
production was found only in a few of them so far.[8] They are also
potentially strong galactic sources of gravitational waves.[9]
Theoretical studies indicate that most supernovae are triggered by one
of two basic mechanisms: the sudden re-ignition of nuclear fusion in a
degenerate star or the sudden gravitational collapse of a massive
star's core. In the first instance, a degenerate white dwarf may
accumulate sufficient material from a binary companion, either through
accretion or via a merger, to raise its core temperature enough to
trigger runaway nuclear fusion, completely disrupting the star. In the
second case, the core of a massive star may undergo sudden
gravitational collapse, releasing gravitational potential energy as a
supernova. While some observed supernovae are more complex than these
two simplified theories, the astrophysical collapse mechanics have
been established and accepted by most astronomers for some time.
Due to the wide range of astrophysical consequences of these events,
astronomers now deem supernova research, across the fields of stellar
and galactic evolution, as an especially important area for
investigation.

The earliest recorded supernova, SN 185, was viewed by Chinese
astronomers in 185 AD. The brightest recorded supernova was SN 1006,
which occurred in 1006 AD and was described in detail by Chinese and
Islamic astronomers.[10] The widely observed supernova SN 1054
produced the Crab Nebula. Supernovae
SN 1572SN 1572 and SN 1604, the latest
to be observed with the naked eye in the
Milky WayMilky Way galaxy, had notable
effects on the development of astronomy in Europe because they were
used to argue against the Aristotelian idea that the universe beyond
the Moon and planets was static and unchanging.[11] Johannes Kepler
began observing
SN 1604SN 1604 at its peak on October 17, 1604, and continued
to make estimates of its brightness until it faded from naked eye view
a year later.[12] It was the second supernova to be observed in a
generation (after
SN 1572SN 1572 seen by
Tycho BraheTycho Brahe in Cassiopeia).[13]
There is some evidence that the youngest galactic supernova, G1.9+0.3,
occurred in the late 19th century, considerably more recently than
Cassiopeia ACassiopeia A from around 1680.[14] Neither supernova was noted at the
time. In the case of G1.9+0.3, high extinction along the plane of the
galaxy could have dimmed the event sufficiently to go unnoticed. The
situation for
Cassiopeia ACassiopeia A is less clear.
InfraredInfrared light echos have
been detected showing that it was a type IIb supernova and was not in
a region of especially high extinction.[15]
Before the development of the telescope, there have only been five
supernovae seen in the last millennium. Compared to a star's entire
history, the visual appearance of a galactic supernova is very brief,
perhaps spanning several months, so that the chances of observing one
is roughly once in a lifetime. Only a tiny fraction of the 100 billion
stars in a typical galaxy have the capacity to become a supernova,
restricted to either having large enough mass or under extraordinarily
rare kinds of binary star in configurations containing white dwarf
stars.[16]
However, observation and discovery of extragalactic supernovae are now
far more common; that started with
SN 1885ASN 1885A in the Andromeda galaxy.
Today, amateur and professional astronomers are finding several
hundreds every year, some when near maximum brightness or others
unrecognised on old astronomical photographs or plates. American
astronomers
Rudolph Minkowski and
Fritz ZwickyFritz Zwicky developed the modern
supernova classification scheme beginning in 1941.[17] During the
1960s, astronomers found that the maximum intensities of supernovae
could be used as standard candles, hence indicators of astronomical
distances.[18] Some of the most distant supernovae recently observed
appeared dimmer than expected. This supports the view that the
expansion of the universe is accelerating.[19] Techniques were
developed for reconstructing supernovae events that have no written
records of being observed. The date of the
Cassiopeia ACassiopeia A supernova
event was determined from light echoes off nebulae,[20] while the age
of supernova remnant
RX J0852.0-4622RX J0852.0-4622 was estimated from temperature
measurements[21] and the gamma ray emissions from the radioactive
decay of titanium-44.[22]
The most luminous supernova ever recorded is ASASSN-15lh. It was first
detected in June 2015 and peaked at 570 billion L☉, which is
twice the bolometric luminosity of any other known supernova.[23]
However, the nature of this supernova continues to be debated and
several alternative explanations have been suggested, e.g. tidal
disruption of a star by a black hole.[24]
Among the earliest detected since time of detonation, and for which
the earliest spectra have been obtained (beginning at 6 hours after
the actual explosion), is the Type II
SN 2013fs (iPTF13dqy) which was
recorded 3 hours after the supernova event on 6 October 2013 by the
Intermediate
Palomar Transient Factory (iPTF). The star is located in
a spiral galaxy named NGC 7610, 160 million light years away in the
constellation of Pegasus.[25][26]
Discovery[edit]
Main article:
History of supernova observationHistory of supernova observation § Telescope
observation

A supernova remnant

Early work on what was originally believed to be simply a new category
of novae was performed during the 1930s by two astronomers named
Walter BaadeWalter Baade and
Fritz ZwickyFritz Zwicky at Mount Wilson Observatory.[27] The
name super-novae was first used during 1931 lectures held at Caltech
by Baade and Zwicky, then used publicly in 1933 at a meeting of the
American Physical Society.[1] By 1938, the hyphen had been lost and
the modern name was in use.[28] Because supernovae are relatively rare
events within a galaxy, occurring about three times a century in the
Milky Way,[29] obtaining a good sample of supernovae to study requires
regular monitoring of many galaxies.
Supernovae in other galaxies cannot be predicted with any meaningful
accuracy. Normally, when they are discovered, they are already in
progress.[30] Most scientific interest in supernovae—as standard
candles for measuring distance, for example—require an observation
of their peak luminosity. It is therefore important to discover them
well before they reach their maximum. Amateur astronomers, who greatly
outnumber professional astronomers, have played an important role in
finding supernovae, typically by looking at some of the closer
galaxies through an optical telescope and comparing them to earlier
photographs.[31]
Toward the end of the 20th century astronomers increasingly turned to
computer-controlled telescopes and CCDs for hunting supernovae. While
such systems are popular with amateurs, there are also professional
installations such as the Katzman Automatic Imaging Telescope.[32]
Recently the
Supernova Early Warning System (SNEWS) project has begun
using a network of neutrino detectors to give early warning of a
supernova in the
Milky WayMilky Way galaxy.[33][34] Neutrinos are particles
that are produced in great quantities by a supernova,[35] and they are
not significantly absorbed by the interstellar gas and dust of the
galactic disk.

"A star set to explode", the SBW1 nebula surrounds a massive blue
supergiant in the Carina Nebula.

SupernovaSupernova searches fall into two classes: those focused on relatively
nearby events and those looking farther away. Because of the expansion
of the universe, the distance to a remote object with a known emission
spectrum can be estimated by measuring its
Doppler shiftDoppler shift (or
redshift); on average, more-distant objects recede with greater
velocity than those nearby, and so have a higher redshift. Thus the
search is split between high redshift and low redshift, with the
boundary falling around a redshift range of z=0.1–0.3[36]—where z
is a dimensionless measure of the spectrum's frequency shift.
High redshift searches for supernovae usually involve the observation
of supernova light curves. These are useful for standard or calibrated
candles to generate Hubble diagrams and make cosmological predictions.
SupernovaSupernova spectroscopy, used to study the physics and environments of
supernovae, is more practical at low than at high redshift.[37][38]
Low redshift observations also anchor the low-distance end of the
Hubble curve, which is a plot of distance versus redshift for visible
galaxies.[39][40] (See also Hubble's law).
Naming convention[edit]

SupernovaSupernova discoveries are reported to the International Astronomical
Union's Central Bureau for Astronomical Telegrams, which sends out a
circular with the name it assigns to that supernova. The name is the
marker SN followed by the year of discovery, suffixed with a one or
two-letter designation. The first 26 supernovae of the year are
designated with a capital letter from A to Z. Afterward pairs of
lower-case letters are used: aa, ab, and so on. Hence, for example, SN
2003C designates the third supernova reported in the year 2003.[41]
The last supernova of 2005 was SN 2005nc, indicating that it was the
367th[nb 1] supernova found in 2005. Since 2000, professional and
amateur astronomers have been finding several hundreds of supernovae
each year (572 in 2007, 261 in 2008, 390 in 2009; 231 in
2013).[42][43]
Historical supernovae are known simply by the year they occurred: SN
185, SN 1006, SN 1054,
SN 1572SN 1572 (called Tycho's Nova) and SN 1604
(Kepler's Star). Since 1885 the additional letter notation has been
used, even if there was only one supernova discovered that year (e.g.
SN 1885A, SN 1907A, etc.) — this last happened with SN 1947A. SN,
for SuperNova, is a standard prefix. Until 1987, two-letter
designations were rarely needed; since 1988, however, they have been
needed every year.
Classification[edit]

Artist's impression of supernova 1993J.[44]

As part of the attempt to understand supernovae, astronomers have
classified them according to their light curves and the absorption
lines of different chemical elements that appear in their spectra. The
first element for division is the presence or absence of a line caused
by hydrogen. If a supernova's spectrum contains lines of hydrogen
(known as the
Balmer seriesBalmer series in the visual portion of the spectrum) it
is classified Type II; otherwise it is Type I. In each of these two
types there are subdivisions according to the presence of lines from
other elements or the shape of the light curve (a graph of the
supernova's apparent magnitude as a function of time).[45][46]

Type I[edit]
Type I supernovae are subdivided on the basis of their spectra, with
Type Ia showing a strong ionised silicon absorption line. Type I
supernovae without this strong line are classified as Type Ib and Ic,
with Type Ib showing strong neutral helium lines and Type Ic lacking
them. The light curves are all similar, although Type Ia are generally
brighter at peak luminosity, but the light curve is not important for
classification of Type I supernovae.
A small number of Type Ia supernovae exhibit unusual features such as
non-standard luminosity or broadened light curves, and these are
typically classified by referring to the earliest example showing
similar features. For example, the sub-luminous
SN 2008haSN 2008ha is often
referred to as SN 2002cx-like or class Ia-2002cx.
A small proportion of type Ic supernovae show highly broadened and
blended emission lines which are taken to indicate very high expansion
velocities for the ejecta. These have been classified as type Ic-BL or
Ic-bl.[48]
Type II[edit]

Light curves are used to classify Type II-P and Type II-L supernovae

The supernovae of Type II can also be sub-divided based on their
spectra. While most Type II supernovae show very broad emission lines
which indicate expansion velocities of many thousands of kilometres
per second, some, such as SN 2005gl, have relatively narrow features
in their spectra. These are called Type IIn, where the 'n' stands for
'narrow'.
A few supernovae, such as SN 1987K and SN 1993J, appear to change
types: they show lines of hydrogen at early times, but, over a period
of weeks to months, become dominated by lines of helium. The term
"Type IIb" is used to describe the combination of features normally
associated with Types II and Ib.[46]
Type II supernovae with normal spectra dominated by broad hydrogen
lines that remain for the life of the decline are classified on the
basis of their light curves. The most common type shows a distinctive
"plateau" in the light curve shortly after peak brightness where the
visual luminosity stays relatively constant for several months before
the decline resumes. These are called Type II-P referring to the
plateau. Less common are Type II-L supernovae that lack a distinct
plateau. The "L" signifies "linear" although the light curve is not
actually a straight line.
Supernovae that do not fit into the normal classifications are
designated peculiar, or 'pec'.[46]
Types III, IV, and V[edit]
Fritz ZwickyFritz Zwicky defined additional supernovae types, although based on a
very few examples that did not cleanly fit the parameters for a Type I
or Type II supernova.
SN 1961i in
NGC 4303NGC 4303 was the prototype and only
member of the Type III supernova class, noted for its broad light
curve maximum and broad hydrogen Balmer lines that were slow to
develop in the spectrum. SN 1961f in NGC 3003 was the prototype and
only member of the Type IV class, with a light curve similar to a Type
II-P supernova, with hydrogen absorption lines but weak hydrogen
emission lines. The Type V class was coined for
SN 1961V in NGC 1058,
an unusual faint supernova or supernova impostor with a slow rise to
brightness, a maximum lasting many months, and an unusual emission
spectrum. The similarity of
SN 1961V to the
Eta CarinaeEta Carinae Great Outburst
was noted.[49] Supernovae in M101 (1909) and M83 (1923 and 1957) were
also suggested as possible Type IV or Type V supernovae.[50]
These types would now all be treated as peculiar Type II supernovae,
of which many more examples have been discovered, although it is still
debated whether
SN 1961V was a true supernova following an LBV
outburst or an impostor.[47]
Current models[edit]

Sequence shows the rapid brightening and slower fading of a supernova
in the galaxy
NGC 1365NGC 1365 (the bright dot close to the upper part of the
galactic center)[51]

The type codes, described above given to supernovae, are taxonomic in
nature: the type number describes the light observed from the
supernova, not necessarily its cause. For example, Type Ia supernovae
are produced by runaway fusion ignited on degenerate white dwarf
progenitors while the spectrally similar Type Ib/c are produced from
massive Wolf–Rayet progenitors by core collapse. The following
summarizes what is currently believed to be the most plausible
explanations for supernovae.
Thermal runaway[edit]
Main article: Type Ia supernova

Formation of a Type Ia supernova

A white dwarf star may accumulate sufficient material from a stellar
companion to raise its core temperature enough to ignite carbon
fusion, at which point it undergoes runaway nuclear fusion, completely
disrupting it. There are three avenues by which this detonation is
theorized to happen: stable accretion of material from a companion,
the collision of two white dwarfs, or accretion that causes ignition
in a shell that then ignites. The dominant mechanism by which Type Ia
supernovae are produced remains unclear.[52] Despite this uncertainty
in how Type Ia supernovae are produced, Type Ia supernovae have very
uniform properties, and are useful standard candles over intergalactic
distances. Some calibrations are required to compensate for the
gradual change in properties or different frequencies of abnormal
luminosity supernovae at high red shift, and for small variations in
brightness identified by light curve shape or spectrum.[53][54]
Normal Type Ia[edit]
There are several means by which a supernova of this type can form,
but they share a common underlying mechanism. If a carbon-oxygen[nb 2]
white dwarf accreted enough matter to reach the
Chandrasekhar limitChandrasekhar limit of
about 1.44 solar masses (M☉)[55] (for a non-rotating star), it would
no longer be able to support the bulk of its mass through electron
degeneracy pressure[56][57] and would begin to collapse. However, the
current view is that this limit is not normally attained; increasing
temperature and density inside the core ignite carbon fusion as the
star approaches the limit (to within about 1%[58]), before collapse is
initiated.[55]
Within a few seconds, a substantial fraction of the matter in the
white dwarf undergoes nuclear fusion, releasing enough energy
(1–7044200000000000000♠2×1044 J)[59] to unbind the star in a
supernova.[60] An outwardly expanding shock wave is generated, with
matter reaching velocities on the order of 5,000–20,000 km/s, or
roughly 3% of the speed of light. There is also a significant increase
in luminosity, reaching an absolute magnitude of −19.3 (or 5 billion
times brighter than the Sun), with little variation.[61]
The model for the formation of this category of supernova is a closed
binary star system. The larger of the two stars is the first to evolve
off the main sequence, and it expands to form a red giant. The two
stars now share a common envelope, causing their mutual orbit to
shrink. The giant star then sheds most of its envelope, losing mass
until it can no longer continue nuclear fusion. At this point it
becomes a white dwarf star, composed primarily of carbon and
oxygen.[62] Eventually the secondary star also evolves off the main
sequence to form a red giant. Matter from the giant is accreted by the
white dwarf, causing the latter to increase in mass. Despite
widespread acceptance of the basic model, the exact details of
initiation and of the heavy elements produced in the catastrophic
event are still unclear.
Type Ia supernovae follow a characteristic light curve—the graph of
luminosity as a function of time—after the event. This luminosity is
generated by the radioactive decay of nickel-56 through cobalt-56 to
iron-56.[61] The peak luminosity of the light curve is extremely
consistent across normal Type Ia supernovae, having a maximum absolute
magnitude of about −19.3. This allows them to be used as a
secondary[63] standard candle to measure the distance to their host
galaxies.[64]
Non-standard Type Ia[edit]
Another model for the formation of Type Ia supernovae involves the
merger of two white dwarf stars, with the combined mass momentarily
exceeding the Chandrasekhar limit.[65] There is much variation in this
type of event,[66] and in many cases there may be no supernova at all,
but it is expected that they will have a broader and less luminous
light curve than the more normal SN Type Ia.
Abnormally bright Type Ia supernovae are expected when the white dwarf
already has a mass higher than the Chandrasekhar limit,[67] possibly
enhanced further by asymmetry,[68] but the ejected material will have
less than normal kinetic energy.
There is no formal sub-classification for the non-standard Type Ia
supernovae. It has been proposed that a group of sub-luminous
supernovae that occur when helium accretes onto a white dwarf should
be classified as Type Iax.[69][70] This type of supernova may not
always completely destroy the white dwarf progenitor and could leave
behind a zombie star.[71]
One specific type of non-standard
Type Ia supernovaType Ia supernova develops hydrogen,
and other, emission lines and gives the appearance of mixture between
a normal Type Ia and a Type IIn supernova. Examples are SN 2002ic and
SN 2005gj. These supernova have been dubbed Type Ia/IIn, Type Ian,
Type IIa and Type IIan.[72]
Core collapse[edit]

The layers of a massive, evolved star just prior to core collapse (Not
to scale)

Very massive stars can undergo core collapse when nuclear fusion
becomes unable to sustain the core against its own gravity; passing
this threshold is the cause of all types of supernova except Type Ia.
The collapse may cause violent expulsion of the outer layers of the
star resulting in a supernova, or the release of gravitational
potential energy may be insufficient and the star may collapse into a
black hole or neutron star with little radiated energy.
Core collapse can be caused by several different mechanisms: electron
capture; exceeding the Chandrasekhar limit; pair-instability; or
photodisintegration.[2][73] When a massive star develops an iron core
larger than the Chandrasekhar mass it will no longer be able to
support itself by electron degeneracy pressure and will collapse
further to a neutron star or black hole.
Electron captureElectron capture by magnesium
in a degenerate O/Ne/Mg core causes gravitational collapse followed by
explosive oxygen fusion, with very similar results. Electron-positron
pair production in a large post-helium burning core removes
thermodynamic support and causes initial collapse followed by runaway
fusion, resulting in a pair-instability supernova. A sufficiently
large and hot stellar core may generate gamma-rays energetic enough to
initiate photodisintegration directly, which will cause a complete
collapse of the core.
The table below lists the known reasons for core collapse in massive
stars, the types of star that they occur in, their associated
supernova type, and the remnant produced. The metallicity is the
proportion of elements other than hydrogen or helium, as compared to
the Sun. The initial mass is the mass of the star prior to the
supernova event, given in multiples of the Sun's mass, although the
mass at the time of the supernova may be much lower.
Type IIn supernovae are not listed in the table. They can potentially
be produced by various types of core collapse in different progenitor
stars, possibly even by Type Ia white dwarf ignitions, although it
seems that most will be from iron core collapse in luminous
supergiants or hypergiants (including LBVs). The narrow spectral lines
for which they are named occur because the supernova is expanding into
a small dense cloud of circumstellar material.[74] It appears that a
significant proportion of supposed Type IIn supernovae are actually
supernova impostors, massive eruptions of LBV-like stars similar to
the Great Eruption of Eta Carinae. In these events, material
previously ejected from the star creates the narrow absorption lines
and causes a shock wave through interaction with the newly ejected
material.[75]

Within a massive, evolved star (a) the onion-layered shells of
elements undergo fusion, forming an iron core (b) that reaches
Chandrasekhar-mass and starts to collapse. The inner part of the core
is compressed into neutrons (c), causing infalling material to bounce
(d) and form an outward-propagating shock front (red). The shock
starts to stall (e), but it is re-invigorated by a process that may
include neutrino interaction. The surrounding material is blasted away
(f), leaving only a degenerate remnant.

When a stellar core is no longer supported against gravity, it
collapses in on itself with velocities reaching 70,000 km/s
(0.23c),[76] resulting in a rapid increase in temperature and density.
What follows next depends on the mass and structure of the collapsing
core, with low mass degenerate cores forming neutron stars, higher
mass degenerate cores mostly collapsing completely to black holes, and
non-degenerate cores undergoing runaway fusion.
The initial collapse of degenerate cores is accelerated by beta decay,
photodisintegration and electron capture, which causes a burst of
electron neutrinos. As the density increases, neutrino emission is cut
off as they become trapped in the core. The inner core eventually
reaches typically 30 km diameter[77] and a density comparable to that
of an atomic nucleus, and neutron degeneracy pressure tries to halt
the collapse. If the core mass is more than about 15 M☉ then
neutron degeneracy is insufficient to stop the collapse and a black
hole forms directly with no supernova.
In lower mass cores the collapse is stopped and the newly formed
neutron core has an initial temperature of about 100 billion kelvin,
6000 times the temperature of the sun's core.[78] At this temperature,
neutrino-antineutrino pairs of all flavors are efficiently formed by
thermal emission. These thermal neutrinos are several times more
abundant than the electron-capture neutrinos.[79] About 1046 joules,
approximately 10% of the star's rest mass, is converted into a
ten-second burst of neutrinos which is the main output of the
event.[77][80] The suddenly halted core collapse rebounds and produces
a shock wave that stalls within milliseconds[81] in the outer core as
energy is lost through the dissociation of heavy elements. A process
that is not clearly understood[update] is necessary to allow the outer
layers of the core to reabsorb around 1044 joules[80] (1 foe) from the
neutrino pulse, producing the visible brightness, although there are
also other theories on how to power the explosion.[77]
Some material from the outer envelope falls back onto the neutron
star, and for cores beyond about 8 M☉ there is sufficient
fallback to form a black hole. This fallback will reduce the kinetic
energy created and the mass of expelled radioactive material, but in
some situations it may also generate relativistic jets that result in
a gamma-ray burst or an exceptionally luminous supernova.
Collapse of massive non-degenerate cores will ignite further fusion.
When the core collapse is initiated by pair instability, oxygen fusion
begins and the collapse may be halted. For core masses of
40–60 M☉, the collapse halts and the star remains intact, but
core collapse will occur again when a larger core has formed. For
cores of around 60–130 M☉, the fusion of oxygen and heavier
elements is so energetic that the entire star is disrupted, causing a
supernova. At the upper end of the mass range, the supernova is
unusually luminous and extremely long-lived due to many solar masses
of ejected 56Ni. For even larger core masses, the core temperature
becomes high enough to allow photodisintegration and the core
collapses completely into a black hole.[82]
Type II[edit]
Main article: Type II supernova

The atypical subluminous Type II SN 1997D

Stars with initial masses less than about eight times the sun never
develop a core large enough to collapse and they eventually lose their
atmospheres to become white dwarfs. Stars with at least 9 M☉
(possibly as much as 12 M☉[83]) evolve in a complex fashion,
progressively burning heavier elements at hotter temperatures in their
cores.[77][84] The star becomes layered like an onion, with the
burning of more easily fused elements occurring in larger
shells.[2][85] Although popularly described as an onion with an iron
core, the least massive supernova progenitors only have
oxygen-neon(-magnesium) cores. These super AGB stars may form the
majority of core collapse supernovae, although less luminous and so
less commonly observed than those from more massive progenitors.[83]
If core collapse occurs during a supergiant phase when the star still
has a hydrogen envelope, the result is a Type II supernova. The rate
of mass loss for luminous stars depends on the metallicity and
luminosity. Extremely luminous stars at near solar metallicity will
lose all their hydrogen before they reach core collapse and so will
not form a Type II supernova. At low metallicity, all stars will reach
core collapse with a hydrogen envelope but sufficiently massive stars
collapse directly to a black hole without producing a visible
supernova.
Stars with an initial mass up to about 90 times the sun, or a little
less at high metallicity, are expected to result in a Type II-P
supernova which is the most commonly observed type. At moderate to
high metallicity, stars near the upper end of that mass range will
have lost most of their hydrogen when core collapse occurs and the
result will be a Type II-L supernova. At very low metallicity, stars
of around 140–250 M☉ will reach core collapse by pair
instability while they still have a hydrogen atmosphere and an oxygen
core and the result will be a supernova with Type II characteristics
but a very large mass of ejected 56Ni and high luminosity.
Type Ib and Ic[edit]
Main article: Type Ib and Ic supernovae

SN 2008D, a Type Ib[86] supernova, shown in
X-rayX-ray (left) and visible
light (right) at the far upper end of the galaxy[87]

These supernovae, like those of Type II, are massive stars that
undergo core collapse. However the stars which become Types Ib and Ic
supernovae have lost most of their outer (hydrogen) envelopes due to
strong stellar winds or else from interaction with a companion.[88]
These stars are known as Wolf–Rayet stars, and they occur at
moderate to high metallicity where continuum driven winds cause
sufficiently high mass loss rates. Observations of Type Ib/c supernova
do not match the observed or expected occurrence of Wolf–Rayet stars
and alternate explanations for this type of core collapse supernova
involve stars stripped of their hydrogen by binary interactions.
Binary models provide a better match for the observed supernovae, with
the proviso that no suitable binary helium stars have ever been
observed.[89] Since a supernova can occur whenever the mass of the
star at the time of core collapse is low enough not to cause complete
fallback to a black hole, any massive star may result in a supernova
if it loses enough mass before core collapse occurs.
Type Ib supernovae are the more common and result from Wolf–Rayet
stars of Type WC which still have helium in their atmospheres. For a
narrow range of masses, stars evolve further before reaching core
collapse to become WO stars with very little helium remaining and
these are the progenitors of Type Ic supernovae.
A few percent of the Type Ic supernovae are associated with gamma-ray
bursts (GRB), though it is also believed that any hydrogen-stripped
Type Ib or Ic supernova could produce a GRB, depending on the
circumstances of the geometry.[90] The mechanism for producing this
type of GRB is the jets produced by the magnetic field of the rapidly
spinning magnetar formed at the collapsing core of the star. The jets
would also transfer energy into the expanding outer shell, producing a
super-luminous supernova.[91][92]
Ultra-stripped supernovae occur when the exploding star has been
stripped (almost) all the way to the metal core, via mass transfer in
a close binary.[93] As a result, very little material is ejected from
the exploding star (~0.1 MSun). In the most extreme cases,
ultra-stripped supernovae can occur in naked metal cores, barely above
the Chandrasekhar mass limit. SN 2005ek[94] might be an observational
example of an ultra-stripped supernova, giving rise to a relatively
dim and fast decaying light curve. The nature of ultra-stripped
supernovae can be both iron core-collapse and electron capture
supernovae, depending on the mass of the collapsing core.
Failed[edit]
Main article: Failed supernova
The core collapse of some massive stars may not result in a visible
supernova. The main model for this is a sufficiently massive core that
the kinetic energy is insufficient to reverse the infall of the outer
layers onto a black hole. These events are difficult to detect, but
large surveys have detected possible candidates.[95][96] The red
supergiant
N6946-BH1N6946-BH1 in
NGC 6946NGC 6946 underwent a modest outburst in March
2009, before fading from view. Only a faint infrared source remains at
the star's location.[97]
Light curves[edit]

Comparative supernova type light curves

A historic puzzle concerned the source of energy that can maintain the
optical supernova glow for months. Although the energy that disrupts
each type of supernovae is delivered promptly, the light curves are
mostly dominated by subsequent radioactive heating of the rapidly
expanding ejecta. Some have considered rotational energy from the
central pulsar. The ejecta gases would dim quickly without some energy
input to keep it hot. The intensely radioactive nature of the ejecta
gases, which is now known to be correct for most supernovae, was first
calculated on sound nucleosynthesis grounds in the late 1960s.[98] It
was not until
SN 1987ASN 1987A that direct observation of gamma-ray lines
unambiguously identified the major radioactive nuclei.[99]
It is now known by direct observation that much of the light curve
(the graph of luminosity as a function of time) after the occurrence
of a Type II Supernova, such as SN 1987A, is explained by those
predicted radioactive decays. Although the luminous emission consists
of optical photons, it is the radioactive power absorbed by the
ejected gases that keeps the remnant hot enough to radiate light. The
radioactive decay of 56Ni through its daughters 56Co to 56Fe produces
gamma-ray photons, primarily of 847keV and 1238keV, that are absorbed
and dominate the heating and thus the luminosity of the ejecta at
intermediate times (several weeks) to late times (several
months).[100] Energy for the peak of the light curve of SN1987A was
provided by the decay of 56Ni to 56Co (half life 6 days) while energy
for the later light curve in particular fit very closely with the 77.3
day half-life of 56Co decaying to 56Fe. Later measurements by space
gamma-ray telescopes of the small fraction of the 56Co and 57Co gamma
rays that escaped the
SN 1987ASN 1987A remnant without absorption confirmed
earlier predictions that those two radioactive nuclei were the power
sources.[99]
The visual light curves of the different supernova types all depend at
late times on radioactive heating, but they vary in shape and
amplitude because of the underlying mechanisms, the way that visible
radiation is produced, the epoch of its observation, and the
transparency of the ejected material. The light curves can be
significantly different at other wavelengths. For example, at
ultraviolet wavelengths there is an early extremely luminous peak
lasting only a few hours corresponding to the breakout of the shock
launched by the initial event, but that breakout is hardly detectable
optically.
The light curves for Type Ia are mostly very uniform, with a
consistent maximum absolute magnitude and a relatively steep decline
in luminosity. Their optical energy output is driven by radioactive
decay of ejected nickel-56 (half life 6 days), which then decays to
radioactive cobalt-56 (half life 77 days). These radioisotopes excite
the surrounding material to incandescence. Studies of cosmology today
rely on 56Ni radioactivity providing the energy for the optical
brightness of supernovae of Type Ia, which are the "standard candles"
of cosmology but whose diagnostic 847keV and 1238keV gamma rays were
first detected only in 2014.[101] The initial phases of the light
curve decline steeply as the effective size of the photosphere
decreases and trapped electromagnetic radiation is depleted. The light
curve continues to decline in the B band while it may show a small
shoulder in the visual at about 40 days, but this is only a hint of a
secondary maximum that occurs in the infra-red as certain ionised
heavy elements recombine to produce infra-red radiation and the ejecta
become transparent to it. The visual light curve continues to decline
at a rate slightly greater than the decay rate of the radioactive
cobalt (which has the longer half life and controls the later curve),
because the ejected material becomes more diffuse and less able to
convert the high energy radiation into visual radiation. After several
months, the light curve changes its decline rate again as positron
emission becomes dominant from the remaining cobalt-56, although this
portion of the light curve has been little-studied.
Type Ib and Ic light curves are basically similar to Type Ia although
with a lower average peak luminosity. The visual light output is again
due to radioactive decay being converted into visual radiation, but
there is a much lower mass of the created nickel-56. The peak
luminosity varies considerably and there are even occasional Type Ib/c
supernovae orders of magnitude more and less luminous than the norm.
The most luminous Type Ic supernovae are referred to as hypernovae and
tend to have broadened light curves in addition to the increased peak
luminosity. The source of the extra energy is thought to be
relativistic jets driven by the formation of a rotating black hole,
which also produce gamma-ray bursts.
The light curves for Type II supernovae are characterised by a much
slower decline than Type I, on the order of 0.05 magnitudes per
day,[102] excluding the plateau phase. The visual light output is
dominated by kinetic energy rather than radioactive decay for several
months, due primarily to the existence of hydrogen in the ejecta from
the atmosphere of the supergiant progenitor star. In the initial
destruction this hydrogen becomes heated and ionised. The majority of
Type II supernovae show a prolonged plateau in their light curves as
this hydrogen recombines, emitting visible light and becoming more
transparent. This is then followed by a declining light curve driven
by radioactive decay although slower than in Type I supernovae, due to
the efficiency of conversion into light by all the hydrogen.[47]
In Type II-L the plateau is absent because the progenitor had
relatively little hydrogen left in its atmosphere, sufficient to
appear in the spectrum but insufficient to produce a noticeable
plateau in the light output. In Type IIb supernovae the hydrogen
atmosphere of the progenitor is so depleted (thought to be due to
tidal stripping by a companion star) that the light curve is closer to
a Type I supernova and the hydrogen even disappears from the spectrum
after several weeks.[47]
Type IIn supernovae are characterised by additional narrow spectral
lines produced in a dense shell of circumstellar material. Their light
curves are generally very broad and extended, occasionally also
extremely luminous and referred to as a superluminous supernova. These
light curves are produced by the highly efficient conversion of
kinetic energy of the ejecta into electromagnetic radiation by
interaction with the dense shell of material. This only occurs when
the material is sufficiently dense and compact, indicating that it has
been produced by the progenitor star itself only shortly before the
supernova occurs.
Large numbers of supernovae have been catalogued and classified to
provide distance candles and test models. Average characteristics vary
somewhat with distance and type of host galaxy, but can broadly be
specified for each supernova type.

Physical properties of supernovae by type[103][104]

Typea
Average peak absolute magnitudeb
Approximate energy (foe)c
Days to peak luminosity
Days from peak to 10% luminosity

Ia
−19
1
approx. 19
around 60

Ib/c (faint)
around −15
0.1
15–25
unknown

Ib
around −17
1
15–25
40–100

Ic
around −16
1
15–25
40–100

Ic (bright)
to −22
above 5
roughly 25
roughly 100

II-b
around −17
1
around 20
around 100

II-L
around −17
1
around 13
around 150

II-P (faint)
around −14
0.1
roughly 15
unknown

II-P
around −16
1
around 15
Plateau then around 50

IInd
around −17
1
12–30 or more
50–150

IIn (bright)
to −22
above 5
above 50
above 100

Notes:

a. ^ Faint types may be a distinct sub-class. Bright types may be a
continuum from slightly over-luminous to hypernovae.
b. ^ These magnitudes are measured in the R band. Measurements in V or
B bands are common and will be around half a magnitude brighter for
supernovae.
c. ^
Order of magnitudeOrder of magnitude kinetic energy. Total electromagnetic radiated
energy is usually lower, (theoretical) neutrino energy much higher.
d. ^ Probably a heterogeneous group, any of the other types embedded
in nebulosity.

A long-standing puzzle surrounding Type II supernovae is why the
remaining compact object receives a large velocity away from the
epicentre;[106] pulsars, and thus neutron stars, are observed to have
high velocities, and black holes presumably do as well, although they
are far harder to observe in isolation. The initial impetus can be
substantial, propelling an object of more than a solar mass at a
velocity of 500 km/s or greater. This indicates an expansion
asymmetry, but the mechanism by which momentum is transferred to the
compact object remains[update] a puzzle. Proposed explanations for
this kick include convection in the collapsing star and jet production
during neutron star formation.
One possible explanation for this asymmetry is a large-scale
convection above the core. The convection can create variations in the
local abundances of elements, resulting in uneven nuclear burning
during the collapse, bounce and resulting expansion.[107]
Another possible explanation is that accretion of gas onto the central
neutron star can create a disk that drives highly directional jets,
propelling matter at a high velocity out of the star, and driving
transverse shocks that completely disrupt the star. These jets might
play a crucial role in the resulting supernova.[108][109] (A similar
model is now favored for explaining long gamma-ray bursts.)
Initial asymmetries have also been confirmed in Type Ia supernovae
through observation. This result may mean that the initial luminosity
of this type of supernova depends on the viewing angle. However, the
expansion becomes more symmetrical with the passage of time. Early
asymmetries are detectable by measuring the polarization of the
emitted light.[110]
Energy output[edit]

The radioactive decays of nickel-56 and cobalt-56 that produce a
supernova visible light curve

Although we are used to thinking of supernovae primarily as luminous
visible events, the electromagnetic radiation they release is almost a
minor side-effect. Particularly in the case of core collapse
supernovae, the emitted electromagnetic radiation is a tiny fraction
of the total energy released during the event.
There is a fundamental difference between the balance of energy
production in the different types of supernova. In Type Ia white dwarf
detonations, most of the energy is directed into heavy element
synthesis and the kinetic energy of the ejecta. In core collapse
supernovae, the vast majority of the energy is directed into neutrino
emission, and while some of this apparently powers the observed
destruction, 99%+ of the neutrinos escape the star in the first few
minutes following the start of the collapse.
Type Ia supernovae derive their energy from a runaway nuclear fusion
of a carbon-oxygen white dwarf. The details of the energetics are
still not fully understood, but the end result is the ejection of the
entire mass of the original star at high kinetic energy. Around half a
solar mass of that mass is 56Ni generated from silicon burning. 56Ni
is radioactive and decays into 56Co by beta plus decay (with a half
life of six days) and gamma rays. 56Co itself decays by the beta plus
(positron) path with a half life of 77 days into stable 56Fe. These
two processes are responsible for the electromagnetic radiation from
Type Ia supernovae. In combination with the changing transparency of
the ejected material, they produce the rapidly declining light
curve.[111]
Core collapse supernovae are on average visually fainter than Type Ia
supernovae, but the total energy released is far higher. In these type
of supernovae, the gravitational potential energy is converted into
kinetic energy that compresses and collapses the core, initially
producing electron neutrinos from disintegrating nucleons, followed by
all flavours of thermal neutrinos from the super-heated neutron star
core. Around 1% of these neutrinos are thought to deposit sufficient
energy into the outer layers of the star to drive the resulting
catastrophe, but again the details cannot be reproduced exactly in
current models. Kinetic energies and nickel yields are somewhat lower
than Type Ia supernovae, hence the lower peak visual luminosity of
Type II supernovae, but energy from the de-ionisation of the many
solar masses of remaining hydrogen can contribute to a much slower
decline in luminosity and produce the plateau phase seen in the
majority of core collapse supernovae.

In some core collapse supernovae, fallback onto a black hole drives
relativistic jets which may produce a brief energetic and directional
burst of gamma rays and also transfers substantial further energy into
the ejected material. This is one scenario for producing high
luminosity supernovae and is thought to be the cause of Type Ic
hypernovae and long duration gamma-ray bursts. If the relativistic
jets are too brief and fail to penetrate the stellar envelope then a
low luminosity gamma-ray burst may be produced and the supernova may
be sub-luminous.
When a supernova occurs inside a small dense cloud of circumstellar
material, it will produce a shock wave that can efficiently convert a
high fraction of the kinetic energy into electromagnetic radiation.
Even though the initial energy was entirely normal the resulting
supernova will have high luminosity and extended duration since it
does not rely on exponential radioactive decay. This type of event may
cause Type IIn hypernovae.
Although pair-instability supernovae are core collapse supernovae with
spectra and light curves similar to Type II-P, the nature after core
collapse is more like that of a giant Type Ia with runaway fusion of
carbon, oxygen, and silicon. The total energy released by the highest
mass events is comparable to other core collapse supernovae but
neutrino production is thought to be very low, hence the kinetic and
electromagnetic energy released is very high. The cores of these stars
are much larger than any white dwarf and the amount of radioactive
nickel and other heavy elements ejected from their cores can be orders
of magnitude higher, with consequently high visual luminosity.
Progenitor[edit]

Play media

Shown in this sped-up artist's impression, is a collection of distant
galaxies, the occasional supernova can be seen. Each of these
exploding stars briefly rivals the brightness of its host galaxy.

The supernova classification type is closely tied to the type of star
at the time of the collapse. The occurrence of each type of supernova
depends dramatically on the metallicity, and hence the age of the host
galaxy.
Type Ia supernovae are produced from white dwarf stars in binary
systems and occur in all galaxy types. Core collapse supernovae are
only found in galaxies undergoing current or very recent star
formation, since they result from short-lived massive stars. They are
most commonly found in Type Sc spirals, but also in the arms of other
spiral galaxies and in irregular galaxies, especially starburst
galaxies.
Type Ib/c and II-L, and possibly most Type IIn, supernovae are only
thought to be produced from stars having near-solar metallicity levels
that result in high mass loss from massive stars, hence they are less
common in older, more-distant galaxies. The table shows the expected
progenitor for the main types of core collapse supernova, and the
approximate proportions that have been observed in the local
neighbourhood.

There are a number of difficulties reconciling modelled and observed
stellar evolution leading up to core collapse supernovae. Red
supergiants are the expected progenitors for the vast majority of core
collapse supernovae, and these have been observed but only at
relatively low masses and luminosities, below about 18 M☉ and
100,000 L☉ respectively. Most progenitors of Type II supernovae
are not detected and must be considerably fainter, and presumably less
massive. It is now proposed that higher mass red supergiants do not
explode as supernovae, but instead evolve back towards hotter
temperatures. Several progenitors of Type IIb supernovae have been
confirmed, and these were K and G supergiants, plus one A
supergiant.[116] Yellow hypergiants or LBVs are proposed progenitors
for Type IIb supernovae, and almost all Type IIb supernovae near
enough to observe have shown such progenitors.[117][118]
Until just a few decades ago, hot supergiants were not considered
likely to explode, but observations have shown otherwise. Blue
supergiants form an unexpectedly high proportion of confirmed
supernova progenitors, partly due to their high luminosity and easy
detection, while not a single Wolf–Rayet progenitor has yet been
clearly identified.[116][119] Models have had difficulty showing how
blue supergiants lose enough mass to reach supernova without
progressing to a different evolutionary stage. One study has shown a
possible route for low-luminosity post-red supergiant luminous blue
variables to collapse, most likely as a Type IIn supernova.[120]
Several examples of hot luminous progenitors of Type IIn supernovae
have been detected: SN 2005gy and SN 2010jl were both apparently
massive luminous stars, but are very distant; and
SN 2009ip had a
highly luminous progenitor likely to have been an LBV, but is a
peculiar supernova whose exact nature is disputed.[116]
The progenitors of Type Ib/c supernovae are not observed at all, and
constraints on their possible luminosity are often lower than those of
known WC stars.[116] WO stars are extremely rare and visually
relatively faint, so it is difficult to say whether such progenitors
are missing or just yet to be observed. Very luminous progenitors have
not been securely identified, despite numerous supernovae being
observed near enough that such progenitors would have been clearly
imaged.[121] Population modelling shows that the observed Type Ib/c
supernovae could be reproduced by a mixture of single massive stars
and stripped-envelope stars from interacting binary systems.[89] The
continued lack of unambiguous detection of progenitors for normal Type
Ib and Ic supernovae may be due to most massive stars collapsing
directly to a black hole without a supernova outburst. Most of these
supernovae are then produced from lower-mass low-luminosity helium
stars in binary systems. A small number would be from rapidly-rotating
massive stars, likely corresponding to the highly-energetic Type Ic-BL
events that are associated with long-duration gamma-ray bursts.[116]
Interstellar impact[edit]
Source of heavy elements[edit]
Main article:
SupernovaSupernova nucleosynthesis

Isolated neutron star in the Small Magellanic Cloud.[122]

Supernovae are the major source of elements heavier than
nitrogen.[123] These elements are produced by nuclear fusion for
nuclei up to 34S, by silicon photodisintegration rearrangement and
quasiequilibrium (see
SupernovaSupernova nucleosynthesis) during silicon
burning for nuclei between 36Ar and 56Ni, and by rapid captures of
neutrons during the supernova's collapse for elements heavier than
iron.
NucleosynthesisNucleosynthesis during silicon burning yields nuclei roughly
1000-100,000 times more abundant than the r-process isotopes heavier
than iron.[124] Supernovae are the most likely, although not
undisputed, candidate sites for the r-process, which is the rapid
capture of neutrons that occurs at high temperature and high density
of neutrons. Those reactions produce highly unstable nuclei that are
rich in neutrons and that rapidly beta decay into more stable forms.
The r-process produces about half of all the heavier isotopes of the
elements beyond iron, including plutonium and uranium.[125] The only
other major competing process for producing elements heavier than iron
is the s-process in large, old, red-giant AGB stars, which produces
these elements slowly over longer epochs and which cannot produce
elements heavier than lead.[126]
Role in stellar evolution[edit]
Main article:
SupernovaSupernova remnant
Remnants of many supernovae consist of a compact object and a rapidly
expanding shock wave of material. This cloud of material sweeps up the
surrounding interstellar medium during a free expansion phase, which
can last for up to two centuries. The wave then gradually undergoes a
period of adiabatic expansion, and will slowly cool and mix with the
surrounding interstellar medium over a period of about 10,000
years.[127]

The
Big BangBig Bang produced hydrogen, helium, and traces of lithium, while
all heavier elements are synthesized in stars and supernovae.
Supernovae tend to enrich the surrounding interstellar medium with
elements other than hydrogen and helium, which usually astronomers
refer to as "metals".
These injected elements ultimately enrich the molecular clouds that
are the sites of star formation.[128] Thus, each stellar generation
has a slightly different composition, going from an almost pure
mixture of hydrogen and helium to a more metal-rich composition.
Supernovae are the dominant mechanism for distributing these heavier
elements, which are formed in a star during its period of nuclear
fusion. The different abundances of elements in the material that
forms a star have important influences on the star's life, and may
decisively influence the possibility of having planets orbiting it.
The kinetic energy of an expanding supernova remnant can trigger star
formation by compressing nearby, dense molecular clouds in space.[129]
The increase in turbulent pressure can also prevent star formation if
the cloud is unable to lose the excess energy.[6]
Evidence from daughter products of short-lived radioactive isotopes
shows that a nearby supernova helped determine the composition of the
Solar SystemSolar System 4.5 billion years ago, and may even have triggered the
formation of this system.[130]
SupernovaSupernova production of heavy elements
over astronomic periods of time ultimately made the chemistry of life
on Earth possible.

Effect on Earth[edit]
Main article: Near-Earth supernova
A near-Earth supernova is a supernova close enough to the Earth to
have noticeable effects on its biosphere. Depending upon the type and
energy of the supernova, it could be as far as 3000 light-years away.
Gamma rays from a supernova would induce a chemical reaction in the
upper atmosphere converting molecular nitrogen into nitrogen oxides,
depleting the ozone layer enough to expose the surface to harmful
ultraviolet solar radiation. This has been proposed as the cause of
the Ordovician–Silurian extinction, which resulted in the death of
nearly 60% of the oceanic life on Earth.[131] In 1996 it was theorized
that traces of past supernovae might be detectable on Earth in the
form of metal isotope signatures in rock strata. Iron-60 enrichment
was later reported in deep-sea rock of the Pacific
Ocean.[132][133][134] In 2009, elevated levels of nitrate ions were
found in Antarctic ice, which coincided with the 1006 and 1054
supernovae. Gamma rays from these supernovae could have boosted levels
of nitrogen oxides, which became trapped in the ice.[135]
Type Ia supernovae are thought to be potentially the most dangerous if
they occur close enough to the Earth. Because these supernovae arise
from dim, common white dwarf stars in binary systems, it is likely
that a supernova that can affect the Earth will occur unpredictably
and in a star system that is not well studied. The closest known
candidate is
IK PegasiIK Pegasi (see below).[136] Recent estimates predict that
a
Type II supernovaType II supernova would have to be closer than eight parsecs (26
light-years) to destroy half of the Earth's ozone layer, and there are
no such candidates closer than about 500 light years.[137]
Milky WayMilky Way candidates[edit]
Main article: List of supernova candidates
The next supernova in the
Milky WayMilky Way will likely be detectable even if
it occurs on the far side of the galaxy. It is likely to be produced
by the collapse of an unremarkable red supergiant and it is very
probable that it will already have been catalogued in infrared surveys
such as 2MASS. There is a smaller chance that the next core collapse
supernova will be produced by a different type of massive star such as
a yellow hypergiant, luminous blue variable, or Wolf–Rayet. The
chances of the next supernova being a Type Ia produced by a white
dwarf are calculated to be about a third of those for a core collapse
supernova. Again it should be observable wherever it occurs, but it is
less likely that the progenitor will ever have been observed. It isn't
even known exactly what a Type Ia progenitor system looks like, and it
is difficult to detect them beyond a few parsecs. The total supernova
rate in our galaxy is estimated to be between 2 and 12 per century,
although we haven't actually observed one for several centuries.[97]

Statistically, the next supernova is likely to be produced from an
otherwise unremarkable red supergiant, but it is difficult to identify
which of those supergiants are in the final stages of heavy element
fusion in their cores and which have millions of years left. The
most-massive red supergiants are expected to shed their atmospheres
and evolve to Wolf–Rayet stars before their cores collapse. All
Wolf–Rayet stars are expected to end their lives from the
Wolf–Rayet phase within a million years or so, but again it is
difficult to identify those that are closest to core collapse. One
class that is expected to have no more than a few thousand years
before exploding are the WO Wolf–Rayet stars, which are known to
have exhausted their core helium.[139] Only eight of them are known,
and only four of those are in the Milky Way.[140]
A number of close or well known stars have been identified as possible
core collapse supernova candidates: the red supergiants
AntaresAntares and
Betelgeuse;[141] the yellow hypergiant Rho Cassiopeiae;[142] the
luminous blue variable
Eta CarinaeEta Carinae that has already produced a
supernova impostor;[143] and the brightest component, a Wolf–Rayet
star, in the Regor or
Gamma VelorumGamma Velorum system,[144] Others have gained
notoriety as possible, although not very likely, progenitors for a
gamma-ray burst; for example WR 104.[145]
Identification of candidates for a
Type Ia supernovaType Ia supernova is much more
speculative. Any binary with an accreting white dwarf might produce a
supernova although the exact mechanism and timescale is still debated.
These systems are faint and difficult to identify, but the novae and
recurrent novae are such systems that conveniently advertise
themselves. One example is U Scorpii.[146] The nearest known Type Ia
supernova candidate is
IK PegasiIK Pegasi (HR 8210), located at a distance of
150 light-years,[147] but observations suggest it will be several
million years before the white dwarf can accrete the critical mass
required to become a Type Ia supernova.[148]
See also[edit]

^ The value is obtained by converting the suffix "nc" from bijective
base-26, with a=1, b=2, c=3, ... z=26. Thus nc=n × 26 + c=14 × 26 +
3=367.
^ For a core primarily composed of oxygen, neon and magnesium, the
collapsing white dwarf will typically form a neutron star. In this
case, only a fraction of the star's mass will be ejected during the
collapse.